Soft contact lenses have so far been prepared typically from homo- or copolymers of acrylic derivatives having OH groups. These materials swell upon absorbing water, have a high degree of transparency and are possessed of good machining qualities, hence being suitable for use in the making of soft contact lenses having a high degree of comfort and adaptability of the eye.
With a view to permitting extended and continuous wear, high-hydration soft contact lenses having an increased water content has recently been proposed. These high-hydration soft contact lenses are made principally from N-vinylpyrrolidone and have high water content. Because of these features, the lens has a sufficient permeability to supply the oxygen necessary for a physiological environment for the cornea, thereby maintaining the active aerobic metabolism of the cornea and contributing significantly to the attainment of physiological safety for the eye.
However, if the water content of the materials principally made from N-vinylpyrrolidone is increased, they experience an abrupt drop in strength and the lens prepared from such materials is subject to durability-associated problems including complete lens failure and development of surface flaws. In particular, since it has poor elongation and is very liable to be damaged by sudden stress, it must be handled with great care. In addition, the materials per se have such a low water retention that the lens prepared therefrom dries up easily to deform or experience other dimensional instabilities.
A process of overcoming such defects is disclosed in Japanese Patent Application No. 134256/85, wherein a high-hydration soft contact lens having high strength is obtained by the use of a copolymer prepared from N,N-dimethylacrylamide, a specific hydrophobic monomer, an unsaturated carboxylic acid having one ethylenic unsaturated bond, and a crosslinking agent. The high-hydration soft contact lens thus obtained has a large tensile strength, but it has small modulus in tension and is soft so that a problem arises in that the base curves, etc. of the lens are apt to be deformed in use.